193 nm water-immersion lithography is currently the state-of-the-art optical lithography used in high volume manufacturing of semiconductors. It will remain so via the additional incorporation of computational lithography and double patterning/double exposure techniques until a next generation lithography technique (such as extreme ultraviolet (EUV) lithography) is available. The imposition of water between the lens and the photoresist in immersion lithography places stringent demands on the resist material. In particular, photoresist components (especially photoacid generators) must be prevented from leaching into the immersion fluid, which would increase defectivity. Scanner manufacturers have established specifications for the maximum rates of PAG leaching in order to protect the lens of the immersion scanner from contamination. In addition, the photoresist surface is required to have high water contact angles in order to contain the water beneath the immersion showerhead of the immersion lithography tool during rapid wafer scanning Beyond a critical velocity, meniscus forces can no longer contain the water at the receding contact line and a trail of residual water is left behind on the wafer (referred to as film pulling). Subsequent evaporation of this residual water has been positively correlated with increasing defects in the printed patterns. Several mechanisms involved in the formation of defects include concentration of extracted materials in the immersion fluid and subsequent deposition on the wafer during drying, localized swelling, and inter-diffusion between the topcoat and photoresist at the spot of droplet evaporation. Collectively, these printed defects are sometimes referred to as watermarks. In addition, the heat of evaporation of the water results in wafer cooling and causes thermal shrinkage of the photoresist layer and overlay problems.
Shedd et al. (Proc. SPIE 6154:61540R (2006)) have shown a correlation between the critical velocity for fluid loss and the static, receding water contact angle (SRCA) of the surface (i.e., hydrophobic surfaces with higher static receding water contact angles allow for faster wafer scanning without film pulling). For more hydrophilic surfaces, the critical velocity is limited by film pulling while for more hydrophobic surfaces the critical velocity is limited by inertial instability.
For standard wafer scan rates of about 500 mm/s, topcoat or photoresist materials should have a static receding water contact angle greater than about 55° to avoid film pulling, with even higher receding water contact angles being advantageous. The required receding water contact angle to avoid film pulling will understandably vary according to the showerhead and fluid management strategy employed by the immersion tool manufacturers and the wafer scan rate, with higher scan rates requiring higher receding water contact angles. Additionally, the values vary slightly depending upon the specific technique employed to measure the water contact angles. Accordingly, these values should be considered only to be general guides; however, the higher the SCRA value of the immersion surface, the faster wafers can be scanned without increasing defectivity. Therefore, immersion surfaces with higher SCRA values will help improve tool throughput and maintain yield by insuring no additional defectivity results from the increased wafer scan rate. Since it is important to optimize both of these parameters for various lithographic processes to be cost effective, it is beneficial to use patterning materials (e.g., topcoats and topcoat-free resists) with high SCRA values.
Additionally, the extremely high advancing water contact angles of early immersion topcoats led to the formation of a class of defects related to micro-bubbles when imaged on early immersion lithography tools. When the advancing water contact angle was too high (greater than about 95°), micro-bubbles of gas could be entrapped in the advancing meniscus at high scan rates; these micro-bubbles acted like micro-lenses and led to circular defects. As such, it was desired that topcoats and photoresists used without topcoats in immersion lithography should have advancing water contact angles less than about 95° to avoid these issues on such tools. Again, the precise advancing water contact angle region in which this bubble defect mechanism occurs differs according to the showerhead design and fluid management strategy applied by different immersion scanner manufacturers. Improvements in showerhead design and scanner tooling have since largely eliminated this type of patterning defect. Therefore, the static advancing water contact angle may no longer be a critical parameter with respect to topcoat performance and defectivity when state-of-the-art immersion tools are employed. However, it is beneficial for materials for immersion lithography to have low contact angle hysteresis (i.e., the difference between the advancing and receding water contact angles should be low).
Conventional photoresists designed for dry 193 nm lithography suffer from low SRCA values (˜50-55° C.) and unacceptable PAG leaching rates. Several materials approaches have been used to make photoresists compatible with immersion lithography and, specifically, to address the PAG leaching and water contact angle issues. The first, and most widely used, method involves coating a protective topcoat material on top of the photoresist to prevent resist component leaching and control water contact angles.
Early topcoats such as TSP-3A from Tokyo Ohka Kogyo (Tokyo, Japan) were based on hydrophobic fluoropolymers. Although these materials possess very large receding contact angles (>100°) with water and enable good lithographic performance, these fluoropolymer topcoats are not soluble in standard aqueous tetramethylammonium hydroxide photoresist developer and, therefore, require an extra topcoat removal step using a fluorinated solvent prior to resist development. These extra process steps and materials increase the cost-of-ownership of this type of topcoat. For at least these reasons, these topcoats are no longer commercially available.
Alkali-soluble topcoats are preferred because they can be removed during the photoresist development step; however, the number of hydrophilic functional groups necessary to impart base-solubility typically limits the SRCAs of these materials to between 55 and 65 degrees. Most acidic groups like carboxylic acid or base-reactive groups like anhydrides are too hydrophilic to impart high SRCA values and low contact angle hysteresis (i.e., the difference between the static advancing and static receding contact angles). For specific examples, see Sundberg et al., Proc. SPIE 6519:65191Q (2007). Fluorine-containing groups such as 1,1,1,3,3,3-hexafluoro-2-hydroxy-propan-2-yl groups (so called hexafluoroalcohol (HFA) or fluoroalcohol) have a sufficiently low pKa that they can dissolve in TMAH developer, yet are relatively hydrophobic and have less detrimental effects on receding contact angles than other alternatives. Due to these advantageous properties, many commercial topcoat materials utilize HFA groups as a relatively non-polar acidic group to impart base solubility. In particular, HFA-functional acrylic polymers based on 1,1,1-trifluoro-2-(trifluoromethyl)-2-hydroxy-pentan-4-yl methacrylate (MA-BTHB-OH) have found widespread use in topcoat materials due to its combination of moderate TMAH dissolution rate and high SRCA (see, Sanders et al., Proc. SPIE 6519:651904 (2007). Copolymerization of MA-BTHB-OH with non-alkali soluble monomers with higher fluorine content can afford copolymers with higher water contact angles; however, only limited quantities of alkali-insoluble comonomers can be incorporated (ca. ˜20%) without rendering the copolymer insoluble in TMAH developer. In addition, it is common to add comonomers (such as those bearing strongly acidic groups) to tune the interaction between the topcoat and the underlying photoresist and, thereby, eliminate any potential profile issues in the final resist pattern (e.g., eliminate T-topping or bridging defects); however, these same comonomers tend to have a detrimental impact on SRCA values. In practice, topcoat materials based on HFA-containing polymers with sufficient dissolution rate (>100 nm/s) in TMAH developer have been unable to progress beyond SRCA values of about 65-70 degrees.
Alternatively, topcoat materials based on sulfonamide-functionalized polymers have been developed. In particular, trifluoromethanesulfonamide groups have been used due to their appropriate pKa, transparency, and hydrophobicity. These materials exhibit higher base dissolution rates but typically much lower SRCA values (see, e.g., poly(MA-BTHB-OH) vs. poly(EATf-MA) in FIG. 1). In addition, the EATf-MA material has a low Tg (˜73° C.) as well, further limiting its utility. The incorporation of cyclic or polycyclic linking groups (see, e.g., Poly(1,4-CHTf-MA) and poly(AMNB-Tf-MA)) has been shown to increase the glass transition temperature and etch resistance but significantly reduces the alkali dissolution rates and does not increase the SRCA values to levels competitive with analogous HFA-based materials. In addition, copolymerization of these faster dissolving sulfonamide-functionalized methacrylates with more hydrophobic monomers cannot match the SRCA values of HFA-based polymers at comparable base dissolution rates.
As such, there is a great need for new materials with high alkali dissolution rates and improved static receding water contact angles for the synthesis of immersion-related patterning materials, such as topcoats. Importantly, these monomers/materials should have high transparency at 193 nm, which precludes the use of heavily absorbing aromatic moieties such as p-toluenesulfonamide groups.
In order to eliminate the additional materials, processes, and cost associated with using a topcoat, topcoat-free resists have been developed which do not require the use of a topcoat to provide good imaging performance with immersion lithography. Immersion-compatible photoresists using hydrophobic base resins are known in the art. In order to increase the SRCA of the base photoresist resin, significant quantities of hydrophobic monomers must be incorporated. This modification can significantly alter the physical, chemical, and lithographic properties of the resist. In practice, these immersion resists have limited SRCA values. The lack of any barrier layer results in high levels of PAG leaching into the immersion fluid when the resist is formulated with conventional PAGs. Therefore, these resists must be formulated with special PAGs with significantly lower water solubility, which in turn limits the freedom of chemists to tune the photoresist performance.
Alternatively, conventional photoresists have been converted to topcoat-free photoresists through the addition of surface-active polymeric additives (typically specially designed fluoropolymers). These additives segregate to the film surface during spin-coating of the photoresist to form a thin enrichment layer and thereby control resist component extraction and water contact angles. A number of types of materials have been designed for use as additives (see, Sanders et al., Proc. SPIE 6519:651904 (2007)). Typically, these additive are classified as topcoat-type (e.g., the additives are alkali-soluble) or resist-type (e.g., the additives have a solubility switch and, typically, become alkali-soluble in the exposed regions after the post-exposure bake due to the action of the photogenerated acid). Many of the topcoat-type additives, however, have acidic moieties such as HFA groups to ensure adequate wetting of the non-exposed regions during alkali development. For the topcoat-type additives, the performance of the HFA-based additives is limited by the relatively low alkali dissolution rates of the high SRCA, HFA-based monomers (e.g., only limited quantities of more hydrophobic comonomers to be incorporated into the polymers before they become insoluble in alkali developer). Sulfonamide-based additives have also been reported; however, these additives suffer from the same limitations discussed previously for topcoat applications. Their lower SRCA values dominate any benefit from their higher dissolution rates, and therefore, HFA-based materials have proven superior. Again, there is a great need for new materials with high alkali dissolution rates and improved static receding water contact angles for the synthesis of polymeric additives for topcoat-free immersion photoresists.